In a battery in which a plurality of storage cells are connected in series, for preventing life reduction and deterioration of discharge capability, it is necessary to suppress variation in voltages (electromotive force) between the storage cells. In particular, in a battery composed of many storage cells, such as an electrical storage device used in electric-powered cars or the like, it is necessary to tightly suppress the variation in voltages between storage cells.
As a system which equalizes the voltages between storage cells, for example, Patent Literature 1 discloses a balancing method as follows: one end of an inductor L is connected to the junction point of series-connected secondary batteries B1 and B2, and the voltage of the battery B1 and the voltage of the battery B2 are equalized by performing during an appropriate period an operation in which a first mode in which current flows in a first closed circuit and a second mode in which current flows in a second closed circuit alternate at short intervals (switching operation); the first closed circuit is formed by connecting the other end of the inductor L to the other end of the battery B1, and the second closed circuit is formed by connecting the other end of the inductor L to the other end of the battery B2 (hereinafter the balancing method disclosed in Patent Literature 1 is referred to as a converter-type method).
Patent Literature 2 discloses assembled cells used for a device such as a notebook computer, the assembled cells including on-off controlled converter circuits and a current control circuit for the purposes of reducing loss of the assembled cells under low load current and cell balancing. Each of the converter circuits is connected in series with a cell, and the input of the converter circuit is the output voltage of the cell. And the output of the cell is connected so that the cell will have been charged. The current control circuit increases or decreases a primary current depending on a load current (hereinafter the balancing method disclosed in Patent Literature 2 is referred to as a transformer-type method).